Memory devices and electronic systems comprising integrated bipolar and FET devices

ABSTRACT

The invention includes BIFETRAM devices. Such devices comprise a bipolar transistor in combination with a field effect transistor (FET) in a three-dimensional stacked configuration. The memory devices can be incorporated within semiconductor-on-insulator (SOI) constructions. The base region of the bipolar device can be physically and electrically connected to one of the source/drain regions of the FET to act as a storage node for the memory cell. The semiconductor material of the SOI constructions can comprise Si/Ge, and the active region of the FET can extend into the Si/Ge. The SOI constructions can be formed over any of a number of substrates, including, for example, semiconductive materials, glass, aluminum oxide, silicon dioxide, metals and/or plastics.

TECHNICAL FIELD

The invention pertains to TFT-based stacked memory devices which include a bipolar transistor in combination with a field effect transistor, and pertains to electronic systems comprising such memory devices. In particular aspects, the memory devices can be considered to be static-memory devices, and accordingly can be incorporated into static random access memory (SRAM) arrays. Such devices can be formed over a versatile array of substrate bases.

BACKGROUND OF THE INVENTION

SOI technology differs from traditional bulk semiconductor technologies in that the active semiconductor material of SOI technologies is typically much thinner than that utilized in bulk technologies. The active semiconductor material of SOI technologies will typically be formed as a thin film over an insulating material (typically oxide), with exemplary thicknesses of the semiconductor film being less than or equal to 2000 Å. In contrast, bulk semiconductor material will typically have a thickness of at least about 200 microns. The thin semiconductor of SOI technology can allow higher performance and lower power consumption to be achieved in integrated circuits than can be achieved with similar circuits utilizing bulk materials.

An exemplary integrated circuit device that can be formed utilizing SOI technologies is a so-called thin film transistor (TFT), with the term “thin film” referring to the thin semiconductor film of the SOI construction. In particular aspects, the semiconductor material of the SOI construction can be silicon, and in such aspects the TFTs can be fabricated using recrystallized amorphous silicon or polycrystalline silicon. The silicon can be supported by an electrically insulative material (such as silicon dioxide), which in turn is supported by an appropriate substrate. Exemplary substrate materials include glass, bulk silicon and metal-oxides (such as, for example, Al₂O₃). If the semiconductor material comprises silicon, the term SOI is occasionally utilized to refer to a silicon-on-insulator construction, rather than the more general concept of a semiconductor-on-insulator construction. However, it is to be understood that in the context of this disclosure the term SOI refers to semiconductor-on-insulator constructions. Accordingly, the semiconductor material of an SOI construction referred to in the context of this disclosure can comprise other semiconductive materials in addition to, or alternatively to, silicon; including, for example, germanium.

A problem associated with conventional TFT constructions is that grain boundaries and defects can limit carrier mobilities. Accordingly, carrier mobilities are frequently nearly an order of magnitude lower than they would be in bulk semiconductor devices. High voltage (and therefore high power consumption), and large areas are utilized for the TFTs, and the TFTs exhibit limited performance. TFTs thus have limited commercial application and currently are utilized primarily for large area electronics.

Various efforts have been made to improve carrier mobility of TFTs. Some improvement is obtained for devices in which silicon is the semiconductor material by utilizing a thermal anneal for grain growth following silicon ion implantation and hydrogen passivation of grain boundaries (see, for example, Yamauchi, N. et al., “Drastically Improved Performance in Poly-Si TFTs with Channel Dimensions Comparable to Grain Size”, IEDM Tech. Digest, 1989, pp. 353-356). Improvements have also been made in devices in which a combination of silicon and germanium is the semiconductor material by optimizing the germanium and hydrogen content of silicon/germanium films (see, for example, King, T. J. et al, “A Low-Temperature (<=550° C.) Silicon-Germanium MOS TFT Technology for Large-Area Electronics”, IEDM Tech. Digest, 1991, pp. 567-570).

Investigations have shown that nucleation, direction of solidification, and grain growth of silicon crystals can be controlled selectively and preferentially by excimer laser annealing, as well as by lateral scanning continuous wave laser irradiation/anneal for recrystallization (see, for example, Kuriyama, H. et al., “High Mobility Poly-Si TFT by a New Excimer Laser Annealing Method for Large Area Electronics”, IEDM Tech. Digest, 1991, pp. 563-566; Jeon, J. H. et al., “A New Poly-Si TFT with Selectively Doped Channel Fabricated by Novel Excimer Laser Annealing”, IEDM Tech. Digest, 2000, pp. 213-216; Kim, C. H. et al., “A New High-Performance Poly-Si TFT by Simple Excimer Laser Annealing on Selectively Floating a Si Layer”, IEDM Tech. Digest, 2001, pp. 753-756; Hara, A. et al, “Selective Single-Crystalline-Silicon Growth at the Pre-Defined Active Regions of TFTs on a Glass by a Scanning CW Layer Irradiation”, IEDM Tech. Digest, 2000, pp. 209-212; and Hara, A. et al., “High Performance Poly-Si TFTs on a Glass by a Stable Scanning CW Laser Lateral Crystallization”, IEDM Tech. Digest, 2001, pp. 747-750). Such techniques have allowed relatively defect-free large crystals to be grown, with resulting TFTs shown to exhibit carrier mobility over 300 cm²N-second.

Another technique which has shown promise for improving carrier mobility is metal-induced lateral recrystallization (MILC), which can be utilized in conjunction with an appropriate high temperature anneal (see, for example, Jagar, S. et al., “Single Grain TFT with SOI CMOS Performance Formed by Metal-Induced-Lateral-Crystallization”, IEDM Tech. Digest, 1999, p. 293-296; and Gu, J. et al., “High Performance Sub-100 nm Si TFT by Pattern-Controlled Crystallization of Thin Channel Layer and High Temperature Annealing”, DRC Conference Digest, 2002, pp. 49-50). A suitable post-recrystallization anneal for improving the film quality within silicon recrystallized by MILC is accomplished by exposing recrystallized material to a temperature of from about 850° C. to about 900° C. under an inert ambient (with a suitable ambient comprising, for example, N₂). MILC can allow nearly single crystal silicon grains to be formed in predefined amorphous-silicon islands for device channel regions. Nickel-induced-lateral-recrystallization can allow device properties to approach those of single crystal silicon.

The carrier mobility of a transistor channel region can be significantly enhanced if the channel region is made of a semiconductor material having a strained crystalline lattice (such as, for example, a silicon/germanium material having a strained lattice, or a silicon material having a strained lattice) formed over a semiconductor material having a relaxed lattice (such as, for example, a silicon/germanium material having a relaxed crystalline lattice). (See, for example, Rim, K. et al., “Strained Si NMOSFETs for High Performance CMOS Technology”, VLSI Tech. Digest, 2001, p. 59-60; Cheng, Z. et al., “SiGe-On-Insulator (SGOI) Substrate Preparation and MOSFET Fabrication for Electron Mobility Evaluation” 2001 IEEE SOI Conference Digest, Oct. 2001, pp.13-14; Huang, L. J. et al., “Carrier Mobility Enhancement in Strained Si-on-Insulator Fabricated by Wafer Bonding”, VLSI Tech. Digest, 2001, pp. 57-58; and Mizuno, T. et al., “High Performance CMOS Operation of Strained-SOI MOSFETs Using Thin Film SiGe-on-Insulator Substrate”, VLSI Tech. Digest, 2002, p.106-107.)

The terms “relaxed crystalline lattice” and “strained crystalline lattice” are utilized to refer to crystalline lattices which are within a defined lattice configuration for the semiconductor material, or perturbed from the defined lattice configuration, respectively. In applications in which the relaxed lattice material comprises silicon/germanium having a germanium concentration of from 10% to 60%, mobility enhancements of 110% for electrons and 60-80% for holes can be accomplished by utilizing a strained lattice material in combination with the relaxed lattice material (see for example, Rim, K. et al., “Characteristics and Device Design of Sub-100 nm Strained SiN and PMOSFETs”, VLSI Tech. Digest, 2002, 00. 98-99; and Huang, L. J. et al., “Carrier Mobility Enhancement in Strained Si-on-Insulator Fabricated by Wafer Bonding”, VLSI Tech. Digest, 2001, pp. 57-58).

Performance enhancements of standard field effect transistor devices are becoming limited with progressive lithographic scaling in conventional applications. Accordingly, strained-lattice-channeled-field effect transistors on relaxed silicon/germanium offers an opportunity to enhance device performance beyond that achieved through conventional lithographic scaling. IBM recently announced the world's fastest communications chip following the approach of utilizing a strained crystalline lattice over a relaxed crystalline lattice (see, for example, “IBM Builds World's Fastest Communications Microchip”, Reuters U.S. Company News, Feb. 25, 2002; and Markoff, J., “IBM Circuits are Now Faster and Reduce Use of Power”, The New York Times, Feb. 25, 2002).

Although various techniques have been developed for substantially controlling nucleation and grain growth processes of semiconductor materials, grain orientation control is lacking. Further, the post-anneal treatment utilized in conjunction with MILC can be unsuitable in applications in which a low thermal budget is desired. Among the advantages of the invention described below is that such can allow substantial control of crystal grain orientation within a semiconductor material, while lowering thermal budget requirements relative to conventional methods. Additionally, the quality of the grown crystal formed from a semiconductor material can be improved relative to that of conventional methods.

In another aspect of the prior art, a static memory cell has been developed which utilizes a reverse base current (RBC) effect of a bipolar transistor for memory storage. (See, for example, “A New Static Memory Cell Based On Reverse Base Current (RBC) Effective Bipolar Transistor”, K. Sakui et al., IEDM Technical Digest, 1988, pages 44-47, which is referred to herein as Sakui et. al.) The memory cell utilizes a field effect transistor in combination with a bipolar transistor, and can enable fabrication of high density SRAM arrays.

SUMMARY OF THE INVENTION

In one aspect, the invention pertains to memory devices comprising a field effect transistor and a bipolar transistor. In one aspect, the invention encompasses a one-device equivalent bipolar and field effect transistor SRAM (BIFETRAM) cell incorporated into a TFT construction. The field effect transistor has a gate and a pair of source/drain regions, and the bipolar transistor has a collector region, emitter region and base region. The collector region and the emitter region of the bipolar transistor are both elevationally above the field effect transistor gate, and the base region of the bipolar transistor is electrically and structurally integrated with one of the source/drain regions of the field effect transistor. The geometry of the lateral bipolar transistor base can be accurately controlled to provide a lower stored charge volume to enhance device performance. The memory device can be incorporated within a semiconductor-on-insulator (SOI) construction, and the stacked bipolar device can be fabricated using an MILC technique adopted to TFT technology. The semiconductor material of the SOI construction can comprise Si/Ge, and the active region of the field effect transistor can extend into the Si/Ge. It can be preferred that an entirety of the active region within the Si/Ge is contained within a single crystal of the Si/Ge. The SOI construction can be formed over any of a number of base substrates, including, for example, semiconductive materials, glass, aluminum oxide, silicon dioxide, metals and/or plastics. In one aspect, the invention enables a TFT-based stacked bipolar transistor integrated on top of a switching FET transistor for development of RBC-SRAM memory arrays over any of a versatile number of substrates. The invention thus can provide density, performance and cost advantages over the previously published memory cell of Sakui et. al.

The invention can also include electronic systems containing static memory devices which include field effect transistors and bipolar transistors.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention are described below with reference to the following accompanying drawings.

FIG. 1 illustrates a circuit schematic of an exemplary BIFETRAM cell according to an aspect of the present invention.

FIG. 2 is a diagrammatic, cross-sectional view of a fragment of a semiconductor construction shown at a preliminary stage of an exemplary process of the present invention

FIG. 3 is a view of the FIG. 2 fragment shown at a processing stage subsequent to that of FIG. 2.

FIG. 4 is a view of the FIG. 2 fragment shown at a processing stage subsequent to that of FIG. 3.

FIG. 5 is a view of the FIG. 2 fragment shown at a processing stage subsequent to that of FIG. 4.

FIG. 6 is a view of the FIG. 2 fragment shown at a processing stage subsequent to that of FIG. 5.

FIG. 7 is a view of the FIG. 2 fragment shown at a processing stage subsequent to that of FIG. 6.

FIG. 8 is an expanded region of the FIG. 7 fragment shown at a processing stage subsequent to that of FIG. 7 in accordance with an exemplary embodiment of the present invention, and shows an n-channel device.

FIG. 9 is a view of the FIG. 8 fragment shown at a processing stage subsequent to that of FIG. 8.

FIG. 10 is a view of an expanded region of FIG. 7 shown at a processing stage subsequent to that of FIG. 7 in accordance with an alternative embodiment relative to that of FIG. 8, and shows a p-channel device.

FIG. 11 is a diagrammatic, fragmentary, cross-sectional view of a BIFETRAM cell according to an exemplary aspect of the present invention.

FIG. 12 is a diagrammatic view of a computer illustrating an exemplary application of the present invention.

FIG. 13 is a block diagram showing particular features of the motherboard of the FIG. 12 computer.

FIG. 14 is a high-level block diagram of an electronic system according to an exemplary aspect of the present invention.

FIG. 15 is a simplified block diagram of an exemplary memory device according to an aspect of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention pertains to memory devices comprising FET and bipolar transistors. In particular aspects, the present invention pertains to a one-device equivalent BIFETRAM cell. The BIFETRAM cell includes an access FET transistor and an integrated, stacked lateral bipolar transistor implemented in a TFT construction. The memory cell utilizes the RBC effect of the lateral bipolar transistor to provide stable memory states. The geometry of the lateral bipolar transistor can be accurately controlled to provide a lower stored charge volume. Additionally, a doping profile can be tailored to provide enhanced performance of the cell.

FIG. 1 is a circuit schematic of a memory cell construction 2 which can be utilized in various aspects of the present invention. Memory cell construction 2 comprises a field effect transistor 3 and a bipolar transistor 4. Field effect transistor 3 comprises a gate 5 and source/drain regions 6 and 7. Gate 5 is electrically connected to a wordline 8 and source/drain region 6 is electrically connected to a bitline 9. The field effect transistor 3 can be referred to as an access transistor.

Bipolar transistor 4 comprises a collector region 11, base region 13, and emitter region 15. Base region 13 is electrically connected to source/drain region 7 of field effect transistor 3, and emitter region 15 is electrically connected to ground 17.

Memory cell construction 2 can be considered to comprise a static memory cell, and can thus correspond to an SRAM construction. The memory cell of memory cell construction 2 can be referred to as a BIFETRAM device to emphasize that the device comprises and uses a bipolar transistor, and that operation of the device utilizes the RBC effect of the bipolar transistor in a similar manner to the description provided in Sakui et. al.

In various embodiments, the bipolar transistor 4 is a lateral n/p/n transistor (and in particular aspects can be a n+/p/n+ transistor or a n+/p−/n+ transistor) integrated on top of the access transistor 3. Accordingly, the PFET source 7 is vertically integrated to form the base of the lateral n/p/n bipolar transistor (such will be described in more detail with reference to FIG. 11 below).

This disclosure pertains to TFT-based SOI constructions incorporating BIFETRAM cells, as well as to the cells themselves. In exemplary aspects, the SOI constructions utilize non-conventional versatile substrates. General aspects of the SOI constructions will next be described with reference to FIGS. 2-10, and this disclosure will then return to a discussion specific to BIFETRAM devices.

FIG. 2 shows a fragment of a semiconductor construction 10 is illustrated at a preliminary processing stage. Construction 10 comprises a base (or substrate) 12 and an insulator layer 14 over the base. Base 12 can comprise, for example, one or more of glass, aluminum oxide, silicon dioxide, metal and plastic. Additionally, and/or alternatively, base 12 can comprise a semiconductor material, such as, for example, a silicon wafer. To aid in interpretation of the claims that follow, the terms “semiconductive substrate” and “semiconductor substrate” are defined to mean any construction comprising semiconductive material, including, but not limited to, bulk semiconductive materials such as a semiconductive wafer (either alone or in assemblies comprising other materials thereon), and semiconductive material layers (either alone or in assemblies comprising other materials). The term “substrate” refers to any supporting structure, including, but not limited to, the semiconductive substrates described above.

Layer 14 comprises an electrically insulative material, and in particular applications can comprise, consist essentially of, or consist of silicon dioxide. In the shown construction, insulator layer 14 is in physical contact with base 12. It is to be understood, however, that there can be intervening materials and layers provided between base 12 and layer 14 in other aspects of the invention (not shown). For example, a chemically passive thermally stable material, such as silicon nitride (Si₃N₄), can be incorporated between base 12 and layer 14. Layer 14 can have a thickness of, for example, from about 200 nanometers to about 500 nanometers, and can be referred to as a buffer layer.

Layer 14 preferably has a planarized upper surface. The planarized upper surface can be formed by, for example, chemical-mechanical polishing.

A layer 16 of semiconductive material is provided over insulator layer 14. In the shown embodiment, semiconductive material layer 16 is formed in physical contact with insulator 14. Layer 16 can have a thickness of, for example, from about 5 nanometers to about 10 nanometers. Layer 16 can, for example, comprise, consist essentially of, or consist of either doped or undoped silicon. If layer 16 comprises, consists essentially of, or consists of doped silicon, the dopant concentration can be from about 10¹⁴ atoms/cm³ to about 10²⁰ atoms/cm³. The dopant can be either n-type or p-type, or a combination of n-type and p-type.

The silicon utilized in layer 16 can be either polycrystalline silicon or amorphous silicon at the processing stage of FIG. 2. It can be advantageous to utilize amorphous silicon in that it is typically easier to deposit a uniform layer of amorphous silicon than to deposit a uniform layer of polycrystalline silicon.

Referring to FIG. 3, material 16 is patterned into a plurality of discrete islands (or blocks) 18. Such can be accomplished utilizing, for example, photoresist (not shown) and photolithographic processing, together with an appropriate etch of material 16.

A capping layer 20 is provided over islands 18 and over portions of layer 14 exposed between the islands. Layer 20 can, for example, comprise, consist essentially of, or consist of one or both of silicon dioxide and silicon. Layer 20 can also comprise multiple layers of silicon dioxide, stress-free silicon oxynitride, and silicon.

After formation of capping layer 20, small voids (nanovoids) and small crystals are formed in the islands 18. The formation of the voids and crystals can be accomplished by ion implanting helium 22 into material 16 and subsequently exposing material 16 to laser-emitted electromagnetic radiation. The helium can aid in formation of the nanovoids; and the nanovoids can in turn aid in crystallization and stress relief within the material 16 during exposure to the electromagnetic radiation. The helium can thus allow crystallization to occur at lower thermal budgets than can be achieved without the helium implantation. The helium is preferably implanted selectively into islands 18 and not into regions between the islands. The exposure of construction 10 to electromagnetic radiation can comprise subjecting the construction to scanned continuous wave laser irradiation while the construction is held at an appropriate elevated temperature (typically from about 300° C. to about 450° C.). The exposure to the electromagnetic radiation can complete formation of single crystal seeds within islands 18. The laser irradiation is scanned along an axis 24 in the exemplary shown embodiment.

The capping layer 20 discussed previously is optional, but can beneficially assist in retaining helium within islands 18 and/or preventing undesirable impurity contamination during the treatment with the laser irradiation.

Referring to FIG. 4, islands 18 are illustrated after voids have been formed therein. Additionally, small crystals (not shown) have also been formed within islands 18 as discussed above.

Capping layer 20 (FIG. 3) is removed, and subsequently a layer 26 of semiconductive material is formed over islands 18. Layer 26 can comprise, consist essentially of, or consist of silicon and germanium; or alternatively can comprise, consist essentially of, or consist of doped silicon/germanium. The germanium concentration within layer 26 can be, for example, from about 10 atomic percent to about 60 atomic percent. In the shown embodiment, layer 26 physically contacts islands 18, and also physically contacts insulator layer 14 in gaps between the islands. Layer 26 can be formed to a thickness of, for example, from about 50 nanometers to about 100 nanometers, and can be formed utilizing a suitable deposition method, such as, for example, plasma-assisted chemical vapor deposition.

A capping layer 28 is formed over semiconductor layer 26. Capping layer 28 can comprise, for example, silicon dioxide. Alternatively, capping layer 28 can comprise, for example, a combination of silicon dioxide and stress-free silicon oxynitride. Capping layer 28 can protect a surface of layer 26 from particles and contaminants that could otherwise fall on layer 26. If the processing of construction 10 occurs in an environment in which particle formation and/or incorporation of contaminants is unlikely (for example, an ultrahigh vacuum environment), layer 28 can be eliminated from the process. Layer 28 is utilized in the patterning of a metal (discussed below). If layer 28 is eliminated from the process, other methods besides those discussed specifically herein can be utilized for patterning the metal.

Referring to FIG. 5, openings 30 are extended through capping layer 28 and to an upper surface of semiconductive material 26. Openings 30 can be formed by, for example, photolithographic processing to pattern a layer of photoresist (not shown) into a mask, followed by a suitable etch of layer 28 and subsequent removal of the photoresist mask.

A layer 32 of metal-containing material is provided within openings 30, and in physical contact with an upper surface of semiconductive material 26. Layer 32 can have a thickness of, for example, less than or equal to about 10 nanometers. The material of layer 32 can comprise, consist essentially of, or consist of, for example, nickel. Layer 32 can be formed by, for example, physical vapor deposition. Layer 32 can be formed to be within openings 30 and not over material 28 (as is illustrated in FIG. 5) by utilizing deposition conditions which selectively form metal-containing layer 32 on a surface of material 26 relative to a surface of material 28. Alternatively, material 32 can be deposited by a substantially non-selective process to form the material 32 over the surface of material 28 as well as over the surface of material 26 within openings 30, and subsequently material 32 can be selectively removed from over surfaces of material 28 while remaining within openings 30. Such selective removal can be accomplished by, for example, chemical-mechanical polishing, and/or by forming a photoresist mask (not shown) over the material 32 within openings 30, while leaving other portions of material 32 exposed, and subsequently removing such other portions to leave only the segments of material 32 within openings 30. The photoresist mask can then be removed.

Oxygen 34 is ion implanted through layers 26 and 28, and into layer 16 to oxidize the material of layer 16. For instance, if layer 16 consists of silicon, the oxygen can convert the silicon to silicon dioxide. Such swells the material of layer 16, and accordingly fills the nanovoids that had been formed earlier. The oxygen preferably only partially oxidizes layer 16, with the oxidation being sufficient to fill all, or at least substantially all, of the nanovoids; but leaving at least some of the seed crystals within layer 16 that had been formed with the laser irradiation discussed previously. In some aspects, the oxidation can convert a lower portion of material 16 to silicon dioxide while leaving an upper portion of material 16 as non-oxidized silicon.

The oxygen ion utilized as implant 34 can comprise, for example, oxygen (O₂) or ozone (O₃). The oxygen ion implant can occur before or after formation of openings 30 and provision of metal-containing layer 32.

Construction 10 is exposed to continuous wave laser irradiation while being held at an appropriate temperature (which can be, for example, from about 300° C. to about 450° C.; or in particular applications can be greater than or equal to 550° C.) to cause transformation of at least some of layer 26 to a crystalline form. The exposure to the laser irradiation comprises exposing the material of construction 10 to laser-emitted electromagnetic radiation scanned along a shown axis 36. Preferably, the axis 36 along which the laser irradiation is scanned is the same axis that was utilized for scanning of laser irradiation in the processing stage of FIG. 3.

The crystallization of material 26 (which can also be referred to as a recrystallization of the material) is induced utilizing metal-containing layer 32, and accordingly corresponds to an application of MILC. The MILC transforms material 26 to a crystalline form and the seed layer provides the crystallographic orientation while undergoing partial oxidation.

The crystal orientation within crystallized layer 26 can originate from the crystals initially formed in islands 18. Accordingly, crystal orientations formed within layer 26 can be controlled through control of the crystal orientations formed within the semiconductive material 16 of islands 18.

The oxidation of part of material 16 which was described previously can occur simultaneously with the MILC arising from continuous wave laser irradiation. Partial oxidation of seed layer 16 facilitates: (1) Ge enrichment into Si—Ge layer 26 (which improves carrier mobility); (2) stress-relief of Si—Ge layer 26; and (3) enhancement of recrystallization of Si—Ge layer 26. The crystallization of material 26 can be followed by an anneal of material 26 at a temperature of, for example, about 900° C. for a time of about 30 minutes, or by an appropriate rapid thermal anneal, to further ensure relaxed, defect-free crystallization of material 26. The annealing option can be dependent on the thermal stability of the material selected for substrate 12.

FIG. 6 shows construction 10 after the processing described above with reference to FIG. 5. Specifically, the voids that had been in material 16 are absent due to the oxidation of material 16. Also, semiconductive material 26 has been transformed into a crystalline material (illustrated diagrammatically by the cross-hatching of material 26 in FIG. 6). Crystalline material 26 can consist of a single large crystal, and accordingly can be monocrystalline. Alternatively, crystalline material 26 can be polycrystalline. If crystalline material 26 is polycrystalline, the crystals of the material will preferably be equal in size or larger than the blocks 18. In particular aspects, each crystal of the polycrystalline material can be about as large as one of the shown islands 18. Accordingly, the islands can be associated in a one-to-one correspondence with crystals of the polycrystalline material.

The shown metal layers 32 are effectively in a one-to-one relationship with islands 18, and such one-to-one correspondence of crystals to islands can occur during the MILC. Specifically, single crystals can be generated relative to each of islands 18 during the MILC process described with reference to FIG. 5. It is also noted, however, that although the metal layers 32 are shown in a one-to-one relationship with the islands in the cross-sectional views of FIGS. 5 and 6, the construction 10 comprising the shown fragment should be understood to extend three dimensionally. Accordingly, the islands 18 and metal layers 32 can extend in directions corresponding to locations into and out of the page relative to the shown cross-sectional view. There can be regions of the construction which are not shown where a metal layer overlaps with additional islands besides the shown islands.

Referring to FIG. 7, layers 28 and 32 (FIG. 6) are removed, and subsequently a layer 40 of crystalline semiconductive material is formed over layer 26. In typical applications, layer 26 will have a relaxed crystalline lattice and layer 40 will have a strained crystalline lattice. As discussed previously, layer 26 will typically comprise both silicon and germanium, with the germanium being present to a concentration of from about 10 atomic percent to about 60 atomic percent. Layer 40 can comprise, consist essentially of, or consist of either doped or undoped silicon; or alternatively can comprise, consist essentially of, or consist of either doped or undoped silicon/germanium. If layer 40 comprises silicon/germanium, the germanium content can be from about 10 atomic percent to about 60 atomic percent.

Strained lattice layer 40 can be formed by utilizing methods similar to those described in, for example, Huang, L. J. et al., “Carrier Mobility Enhancement in Strained Si-on-Insulator Fabricated by Wafer Bonding”, VLSI Tech. Digest, 2001, pp. 57-58; and Cheng, Z. et al., “SiGe-On-Insulator (SGOI) Substrate Preparation and MOSFET Fabrication for Electron Mobility Evaluation” 2001 IEEE SOI Conference Digest, October 2001, pp.13-14.

Strained lattice layer 40 can be large polycrystalline or monocrystalline. If strained lattice layer 40 is polycrystalline, the crystals of layer 40 can be large and in a one-to-one relationship with the large crystals of a polycrystalline relaxed crystalline layer 26. Strained lattice layer 40 is preferably monocrystalline over the individual blocks 18.

The strained crystalline lattice of layer 40 can improve mobility of carriers relative to the material 26 having a relaxed crystalline lattice. However, it is to be understood that layer 40 is optional in various aspects of the invention.

Each of islands 18 can be considered to be associated with a separate active region 42, 44 and 46. The active regions can be separated from one another by insulative material subsequently formed through layers 26 and 40 (not shown). For instance, a trenched isolation region can be formed through layers 26 and 40 by initially forming a trench extending through layers 26 and 40 to insulative material 14, and subsequently filling the trench with an appropriate insulative material such as, for example, silicon dioxide.

As discussed previously, crystalline material 26 can be a single crystal extending across an entirety of the construction 10 comprising the shown fragment, and accordingly extending across all of the shown active regions. Alternatively, crystalline material 26 can be polycrystalline. If crystalline material 26 is polycrystalline, the single crystals of the polycrystalline material will preferably be large enough so that only one single crystal extends across a given active region. In other words, active region 42 will preferably comprise a single crystal of material 26, active region 44 will comprise a single crystal of the material, and active region 46 will comprise a single crystal of the material, with the single crystals being separate and discrete relative to one another.

FIG. 8 shows an expanded view of active region 44 at a processing stage subsequent to that of FIG. 7, and specifically shows a transistor device 50 associated with active region 44 and supported by crystalline material 26.

Transistor device 50 comprises a dielectric material 52 formed over strained lattice 40, and a gate 54 formed over dielectric material 52. Dielectric material 52 typically comprises silicon dioxide, and gate 54 typically comprises a stack including an appropriate conductive material, such as, for example, conductively-doped silicon and/or metal.

A channel region 56 is beneath gate 54, and in the shown construction extends across strained crystalline lattice material 40. The channel region may also extend into relaxed crystalline lattice material 26 (as shown). Channel region 56 is doped with a p-type dopant.

Transistor construction 50 additionally comprises source/drain regions 58 which are separated from one another by channel region 56, and which are doped with n-type dopant to an n⁺ concentration (typically, a concentration of at least 10²¹ atoms/cm³). In the shown construction, source/drain regions 58 extend across strained lattice layer 40 and into relaxed lattice material 26. Although source/drain regions 58 are shown extending only partially through relaxed lattice layer 26, it is to be understood that the invention encompasses other embodiments (not shown) in which the source/drain regions extend all the way through relaxed material 26 and to material 16.

Channel region 56 and source/drain regions 58 can be formed by implanting the appropriate dopants into crystalline materials 26 and 40. The dopants can be activated by rapid thermal activation (RTA), which can aid in keeping the thermal budget low for fabrication of field effect transistor 50.

An active region of transistor device 50 extends across source/drain regions 58 and channel region 56. Preferably the portion of the active region within crystalline material 26 is associated with only one single crystal of material 26. Such can be accomplished by having material 26 be entirely monocrystalline. Alternatively, material 26 can be polycrystalline and comprise an individual single grain which accommodates the entire portion of the active region that is within material 26. The portion of strained lattice material 40 that is encompassed by the active region is preferably a single crystal, and can, in particular aspects, be considered an extension of the single crystal of the relaxed lattice material 26 of the active region.

Crystalline materials 40 and 26 can, together with any crystalline structures remaining in material 16, have a total thickness of less than or equal to about 2000 Å. Accordingly the crystalline material can correspond to a thin film formed over an insulative material. The insulative material can be considered to be insulative layer 14 alone, or a combination of insulative layer 14 and oxidized portions of material 16.

The transistor structure 50 of FIG. 8 corresponds to an n-type field effect transistor (NFET), and in such construction it can be advantageous to have strained crystalline material 40 consist of a strained silicon material having appropriate dopants therein. The strained silicon material can improve mobility of electrons through channel region 56, which can improve performance of the NFET device relative to a device lacking the strained silicon lattice. Although it can be preferred that strained lattice material 40 comprise silicon in an NFET device, it is to be understood that the strained lattice can also comprise other semiconductive materials. A strained silicon lattice can be formed by various methods. For instance, strained silicon could be developed by various means and lattice 40 could be created by lattice mismatch with other materials or by geometric conformal lattice straining on another substrate (mechanical stress).

As mentioned above, strained lattice 40 can comprise other materials alternatively to, or additionally to, silicon. The strained lattice can, for example, comprise a combination of silicon and germanium. There can be advantages to utilizing the strained crystalline lattice comprising silicon and germanium relative to structures lacking any strained lattice. However, it is generally most preferable if the strained lattice consists of silicon alone (or doped silicon), rather than a combination of silicon and germanium for an NFET device.

A pair of sidewall spacers 60 are shown formed along sidewalls of gate 54, and an insulative mass 62 is shown extending over gate 54 and material 40. Conductive interconnects 63 and 64 extend through the insulative mass 62 to electrically connect with source/drain regions 58. Interconnects 63 and 64 can be utilized for electrically connecting transistor construction 50 with other circuitry external to transistor construction 50. Such other circuitry can include, for example, a bitline and a capacitor in applications in which construction 50 is incorporated into dynamic random access memory (DRAM).

FIG. 9 shows construction 10 at a processing stage subsequent to that of FIG. 8, and shows a capacitor structure 100 formed over and in electrical contact with conductive interconnect 64. The shown capacitor structure extends across gate 54 and interconnect 63.

Capacitor construction 100 comprises a first capacitor electrode 102, a second capacitor electrode 104, and a dielectric material 106 between capacitor electrodes 102 and 104. Capacitor electrodes 102 and 104 can comprise any appropriate conductive material, including, for example, conductively-doped silicon. In particular aspects, electrodes 102 and 104 will each comprise n-type doped silicon, such as, for example, polycrystalline silicon doped to a concentration of at least about 10²¹ atoms/cm³ with n-type dopant. In a particular aspect of the invention, electrode 102, conductive interconnect 64 and the source/drain region 58 electrically connected with interconnect 64 comprise, or consist of, n-type doped semiconductive material. Accordingly, n-type doped semiconductive material extends from the source/drain region, through the interconnect, and through the capacitor electrode.

Dielectric material 106 can comprise any suitable material, or combination of materials. Exemplary materials suitable for dielectric 106 are high dielectric constant materials including, for example, silicon nitride, aluminum oxide, TiO₂, Ta₂O₅, ZrO₂, etc.

The conductive interconnect 63 is in electrical connection with a bitline 108. Top capacitor electrode 104 is shown in electrical connection with an interconnect 110, which in turn connects with a reference voltage 112, which can, in particular aspects, be ground. The construction of FIG. 9 can be considered a DRAM cell, and such can be incorporated into an electronic system (such as, for example, a computer system) as a memory device.

FIG. 10 shows construction 10 at a processing stage subsequent to that of FIG. 7 and alternative to that described previously with reference to FIG. 8. In referring to FIG. 10, similar numbering will be used as is used above in describing FIG. 8, where appropriate.

A transistor construction 70 is shown in FIG. 10, and such construction differs from the construction 50 described above with reference to FIG. 8 in that construction 70 is a p-type field effect transistor (PFET) rather than the NFET of FIG. 8. Transistor device 70 comprises an n-type doped channel region 72 and p⁺-doped source/drain regions 74. In other words, the channel region and source/drain regions of transistor device 70 are oppositely doped relative to the channel region and source/drain regions described above with reference to the NFET device 50 of FIG. 8.

The strained crystalline lattice material 40 of the PFET device 70 can consist of appropriately doped silicon, or consist of appropriately doped silicon/germanium. It can be most advantageous if the strained crystalline lattice material 40 comprises appropriately doped silicon/germanium in a PFET construction, in that silicon/germanium can be a more effective carrier of holes with higher mobility than is silicon without germanium.

The transistor devices discussed above (NFET device 50 of FIG. 8, and PFET device 70 of FIG. 10) can be utilized in numerous constructions in addition to the construction described above with reference to FIG. 9. FIG. 11 illustrates another exemplary application for the transistor devices. Specifically, FIG. 11 is a cross-sectional view of an exemplary BIFETRAM construction 200 in accordance with an aspect of the present invention. In referring to construction 200, similar numbering will be used as was used above in describing the constructions of FIGS. 1-10, where appropriate.

Construction 200 comprises the substrate base 12 and insulative material 14 described previously with reference to FIGS. 2-10. Substrate base 12 can accordingly comprise, for example, one or more of glass, aluminum oxide, silicon dioxide, metal and plastic; and insulative material 14 can comprise, for example, silicon dioxide.

The layers 16, 26 and 40 described previously with reference to FIGS. 2-10 are formed over insulative material 14. Layer 16 can thus comprise, consist essentially of, or consist of one or both of silicon and silicon dioxide; layer 26 can comprise, consist essentially of, or consist of appropriately doped Si/Ge; and layer 40 can comprise, consist essentially of, or consist of appropriately doped silicon and/or Si/Ge. Further, layer 26 can comprise a crystalline lattice, and in particular aspects can comprise a relaxed crystalline lattice. Also, layer 40 can comprise a crystalline lattice, and in particular aspects can comprise a strained crystalline lattice. Layers 26 and 40 can together have a thickness of less than or equal to 2,000 Angstroms, and can be considered a thin film of semiconductor material formed over insulative material 14. Accordingly, materials 26, 40 and 14 can be together considered to comprise a semiconductor-on-insulator construction. As discussed above, layer 16 can comprise one or both of insulative material and semiconductor material, and accordingly can be a component of the semiconductor portion of the semiconductor-on-insulator construction, and/or can be a component of the insulative part of the SOI construction.

A field effect transistor device 70 is formed over and within layers 26 and 40. Device 70 includes a conductive gate 54, which is separated from layer 40 by an insulative material 52. Sidewall spacers 60 extend along sidewalls of gate 54. Source/drain regions 6 and 7 extend into materials 26 and 40 proximate gate 54, and are separated from one another by a channel region 72. Field effect transistor 70 has an identical construction to the transistor described previously with reference to FIG. 10. It is noted that the source/drain regions of FIG. 10 are labeled as 74, whereas those of FIG. 11 are labeled as 6 and 7. The numbering of FIG. 11 is chosen to indicate that the source/drain regions of FIG. 11 can correspond to the source/drain regions 6 and 7 shown in the circuit diagram of FIG. 1.

Transistor device 70 can have an active region which extends within layers 26 and 40, and which comprises source/drain regions 6 and 7. Preferably, the portion of the active region within crystalline material 26 will be entirely contained within a single crystal of material 26, and the portion of the active region within material 40 will be entirely contained within a single crystal of material 40.

Source/drain region 6 is electrically connected to a bitline 9, which can correspond to the bitline 9 described with reference to FIG. 1. Contact associated with bitline 9 to source/drain regions 6 of adjacent cells can be shared to improve cell density (not shown).

Source/drain region 7 is electrically connected to a pillar 202 of conductive material. In the shown embodiment, pillar 202 comprises conductively-doped semiconductive material, and specifically comprises p-type doped semiconductive material. The semiconductive material of pillar 202 can comprise, consist essentially of, or consist of, for example, silicon and/or Si/Ge. The pillar extends and integrates to the base region of a bipolar transistor device 204 which comprises an n-type doped collector region 206, a p-type doped base region 208, and an n-type doped emitter region 210. In the shown exemplary embodiment, base region 208 is doped to a (p−) dopant level and a portion of the pillar beneath the base region is doped to a (p) dopant level. Such can be accomplished by forming pillar 202 and base 208 of non-doped semiconductive material over conductively-doped source/region 7, and subsequently out-diffusing dopant from source/drain region 7 into pillar 202 and base 208. A gradient of dopant concentration is thus formed through pillar 202 to base 208, with a lower portion of the pillar being doped to a (p) concentration and the upper portion of the pillar and base being doped to a (p−) concentration.

Collector region 206 and emitter region 210 are shown doped to n+ concentrations. Such can be accomplished by forming a layer of semiconductive material in the configuration of bipolar transistor device 204, and subsequently implanting n type dopant into regions 206 and 210 to form the collector and emitter regions of the bipolar transistor device.

Collector region 206 is in electrical contact with VDD 212, and emitter region 210 is in electrical contact with ground 17. The emitter region 210 and the contact 17 associated with it could be shared between adjacent cells (not shown) to improve cell density.

The shown construction has an entirety of the collector region 206 and emitter region 210 of the bipolar device elevationally above a top surface of conductive gate 54 of the field effect transistor 70. Additionally, pillar 202 extends vertically, and in the shown exemplary aspect extends perpendicularly relative to an upper planar surface of layer 40, and integrates to form the TFT bipolar base region 208 of well-defined small volume. This feature can result in a faster performance bipolar device as compared to a conventional bulk silicon bipolar device.

Insulative material 62 is formed over and around various components of construction 200 to support bipolar device 204 over field effect transistor device 70.

Source/drain regions 6 and 7 can be referred to as being primarily p-type doped regions, with the term “primarily” indicating that the majority of the source/drain regions is p-type doped. It is to be understood, however, that the source/drain regions can comprise components which are n-type doped, such as, for example, halo components.

In operation, the bipolar transistor of construction 200 can have a negative resistance effect as well as a reverse base current effect similar to a bulk bipolar device. The bipolar device of construction 200 can be similar to conventional bipolar devices, yet have superior performance by using a very small relative size of the body (base portion 208) relative to the rest of the device to allow a volume of charge storage to be small compared to conventional devices. The bipolar device can be utilized as a switch, with field effect transistor gate 70 enabling the switch to be turned off and on quickly.

FIG. 11 shows various different dopant levels, and utilizes some or all of the designations p+, p , p−, n−, n and n+ to distinguish the levels. The difference in dopant concentration between the regions identified as being p+, p, and p− are typically as follows. A p+ region has a dopant concentration of at least about 10^(°) atoms/cm³, a p region has a dopant concentration of from about 10¹⁴ to about 10¹⁸ atoms/cm³, and a p− region has a dopant concentration in the order of or less than 10¹⁶ atoms/cm³. It is noted that regions identified as being n−, n and n+ will have dopant concentrations similar to those described above relative to the p−, p and p+ regions respectively, except, of course, the n regions will have an opposite-type conductivity enhancing dopant therein than do the p regions.

The p+, p, and p− dopant levels are shown in the drawing only to illustrate differences in dopant concentration. It is noted that the term “p” is utilized herein to refer to both a dopant type and a relative dopant concentration. To aid in interpretation of this specification and the claims that follow, the term “p” is to be understood as referring only to dopant type, and not to a relative dopant concentration, except when it is explicitly stated that the term “p” refers to a relative dopant concentration. Accordingly, for purposes of interpreting this disclosure and the claims that follow, it is to be understood that the term “p-type doped” refers to a dopant type of a region and not a relative dopant level. Thus, a p-type doped region can be doped to any of the p+, p, and p− dopant levels discussed above. Similarly, an n-type doped region can be doped to any of the n+, n, and n− dopant levels discussed above.

The read/write operations of the BIFETRAM cell of FIG. 11 can be identical to those described by Sakui et. al. for the memory cell shown therein, and accordingly will not be elaborated in this disclosure.

FIG. 12 illustrates generally, by way of example, but not by way of limitation, an embodiment of a computer system 400 according to an aspect of the present invention. Computer system 400 includes a monitor 401 or other communication output device, a keyboard 402 or other communication input device, and a motherboard 404. Motherboard 404 can carry a microprocessor 406 or other data processing unit, and at least one memory device 408. Memory device 408 can comprise various aspects of the invention described above, including, for example, the DRAM unit cell described with reference to FIG. 8 or the BIFETRAM memory cell described with reference to FIG. 11. Memory device 408 can comprise an array of memory cells, and such array can be coupled with addressing circuitry for accessing individual memory cells in the array. Further, the memory cell array can be coupled to a read circuit for reading data from the memory cells. The addressing and read circuitry can be utilized for conveying information between memory device 408 and processor 406. Such is illustrated in the block diagram of the motherboard 404 shown in FIG. 13. In such block diagram, the addressing circuitry is illustrated as 410 and the read circuitry is illustrated as 412.

In particular aspects of the invention, memory device 408 can correspond to a memory module. For example, single in-line memory modules (SIMMs) and dual in-line memory modules (DIMMs) may be used in the implementation which utilize the teachings of the present invention. The memory device can be incorporated into any of a variety of designs which provide different methods of reading from and writing to memory cells of the device. One such method is the page mode operation. Page mode operations in a DRAM are defined by the method of accessing a row of a memory cell arrays and randomly accessing different columns of the array. Data stored at the row and column intersection can be read and output while that column is accessed.

An alternate type of device is the extended data output (EDO) memory which allows data stored at a memory array address to be available as output after the addressed column has been closed. This memory can increase some communication speeds by allowing shorter access signals without reducing the time in which memory output data is available on a memory bus. Other alternative types of devices include SDRAM, DDR SDRAM, SLDRAM, VRAM and Direct RDRAM, as well as others such as SRAM or Flash memories.

FIG. 14 illustrates a simplified block diagram of a high-level organization of various embodiments of an exemplary electronic system 700 of the present invention. System 700 can correspond to, for example, a computer system, a process control system, or any other system that employs a processor and associated memory. Electronic system 700 has functional elements, including a processor or arithmetic/logic unit (ALU) 702, a control unit 704, a memory device unit 706 and an input/output (I/O) device 708. Generally, electronic system 700 will have a native set of instructions that specify operations to be performed on data by the processor 702 and other interactions between the processor 702, the memory device unit 706 and the I/O devices 708. The control unit 704 coordinates all operations of the processor 702, the memory device 706 and the I/O devices 708 by continuously cycling through a set of operations that cause instructions to be fetched from the memory device 706 and executed. In various embodiments, the memory device 706 includes, but is not limited to, random access memory (RAM) devices, read-only memory (ROM) devices, and peripheral devices such as a floppy disk drive and a compact disk CD-ROM drive. One of ordinary skill in the art will understand, upon reading and comprehending this disclosure, that any of the illustrated electrical components are capable of being fabricated to include DRAM cells and BIFETRAM cells in accordance with various aspects of the present invention.

FIG. 15 is a simplified block diagram of a high-level organization of various embodiments of an exemplary electronic system 800. The system 800 includes a memory device 802 that has an array of memory cells 804, address decoder 806, row access circuitry 808, column access circuitry 810, read/write control circuitry 812 for controlling operations, and input/output circuitry 814. The memory device 802 further includes power circuitry 816, and sensors 820, such as current sensors for determining whether a memory cell is in a low-threshold conducting state or in a high-threshold non-conducting state. The illustrated power circuitry 816 includes power supply circuitry 880, circuitry 882 for providing a reference voltage, circuitry 884 for providing the first wordline with pulses, circuitry 886 for providing the second wordline with pulses, and circuitry 888 for providing the bitline with pulses. The system 800 also includes a processor 822, or memory controller for memory accessing.

The memory device 802 receives control signals 824 from the processor 822 over wiring or metallization lines. The memory device 802 is used to store data which is accessed via I/O lines. It will be appreciated by those skilled in the art that additional circuitry and control signals can be provided, and that the memory device 802 has been simplified to help focus on the invention. At least one of the processor 822 or memory device 802 can include a DRAM cell or a BIFETRAM cell of the type described previously in this disclosure.

The various illustrated systems of this disclosure are intended to provide a general understanding of various applications for the circuitry and structures of the present invention, and are not intended to serve as a complete description of all the elements and features of an electronic system using memory cells in accordance with aspects of the present invention. One of the ordinary skill in the art will understand that the various electronic systems can be fabricated in single-package processing units, or even on a single semiconductor chip, in order to reduce the communication time between the processor and the memory device(s).

Applications for memory cells can include electronic systems for use in memory modules, device drivers, power modules, communication modems, processor modules, and application-specific modules, and may include multilayer, multichip modules. Such circuitry can further be a subcomponent of a variety of electronic systems, such as a clock, a television, a cell phone, a personal computer, an automobile, an industrial control system, an aircraft, and others.

In compliance with the statute, the invention has been described in language more or less specific as to structural and methodical features. It is to be understood, however, that the invention is not limited to the specific features shown and described, since the means herein disclosed comprise preferred forms of putting the invention into effect. The invention is, therefore, claimed in any of its forms or modifications within the proper scope of the appended claims appropriately interpreted in accordance with the doctrine of equivalents. 

1. A memory device comprising: a crystalline layer separated from a substrate by an insulative material; the crystalline layer being less than or equal to about 2000 Å thick and comprising a material which includes silicon and germanium; a field effect transistor having a transistor gate and a pair of source/drain regions; a bipolar transistor having a collector region, emitter region and base region; the collector region and emitter region both being elevationally above the transistor gate; the base region electrically connecting with one of the source/drain regions of the field effect transistor; and wherein the field effect transistor is over the crystalline layer and has an active region extending into the crystalline layer; at least a portion of the source/drain regions is comprised by the active region within the crystalline layer; the entirety of the active region within the crystalline layer being contained within a single crystal of the material which includes silicon and germanium.
 2. The construction of claim 1 wherein the material which includes silicon and germanium comprises from about 10 to about 60 atomic percent germanium.
 3. The memory device of claim 1 wherein the crystalline layer is polycrystalline.
 4. The memory device of claim 1 wherein the crystalline layer is monocrystalline.
 5. The memory device of claim 1 wherein the crystalline layer has a relaxed crystalline lattice, and further comprising a strained crystalline lattice layer between the relaxed crystalline lattice and the field effect transistor gate.
 6. The memory device of claim 5 wherein the strained crystalline lattice includes silicon.
 7. The memory device of claim 5 wherein the strained crystalline lattice includes silicon and germanium.
 8. The construction of claim 1 wherein the substrate comprises a semiconductive material.
 9. The construction of claim 1 wherein the substrate comprises glass.
 10. The construction of claim 1 wherein the substrate comprises aluminum oxide.
 11. The construction of claim 1 wherein the substrate comprises silicon dioxide.
 12. The construction of claim 1 wherein the substrate comprises a metal.
 13. The construction of claim 1 wherein the substrate comprises a plastic.
 14. An electronic system, the electronic system comprising a memory device, the memory device including: an array of memory cells, at least some of the memory cells including a field effect transistor and a bipolar transistor; the field effect transistors having transistor gates and active regions comprising source/drain regions; the active regions extending into a crystalline Si/Ge layer; the bipolar transistors having collector regions, emitter regions and base regions; the collector regions and emitter regions being above the transistor gates; the base regions electrically connecting with source/drain regions of the field effect transistors; addressing circuitry coupled to the array of memory cells for accessing individual memory cell in the array of memory cells; a read circuit coupled to the memory cell array for reading data from memory cells in the array of memory cells; and wherein the crystalline layer has a relaxed crystalline lattice, and further comprising a strained crystalline lattice layer between the relaxed crystalline lattice and the transistor gates.
 15. The electronic system of claim 14 wherein the strained crystalline lattice includes silicon.
 16. The electronic system of claim 14 wherein the strained crystalline lattice includes silicon and germanium. 